In a large soup pot, bring the fish stock or water to a gentle boil over medium-high heat.

Add the whole peeled onion, diced potatoes, sliced carrots, bay leaves, black peppercorns, and sea salt to the boiling liquid.

Reduce the heat to medium-low, cover the pot with a lid, and simmer for 15 minutes, or until the potatoes and carrots are easily pierced with a fork.

Carefully lower the chunks of white fish into the pot, ensuring they are submerged in the broth. Simmer very gently for another 5 to 7 minutes, or until the fish is opaque and flakes easily.

Remove the pot from the heat. Use a slotted spoon to carefully fish out and discard the whole onion and the bay leaves.

Stir in the fresh chopped dill and the unsalted butter, allowing the butter to melt completely and lightly enrich the clear broth.

Ladle the hot soup into deep bowls, ensuring an even distribution of fish and tender vegetables, and serve immediately alongside dark rye bread.